1. Field of the Invention
The present invention relates to an optical fiber recoating device for reapplying a resin coating to an area along an optical fiber where the sheath has been removed. More specifically, the present invention relates to an improvement in the molds employed in the device.
2. Description of the Related Art
In order to protect an optical fiber, it is necessary to reapply resin to an area (a bare fiber portion) where the sheath thereof has been removed, which can occur when connecting optical fibers using heat fusion, for example. In other words, the optical fiber must be recoated.
A number of devices have been proposed as conventional recoating devices. One example of these is a device of the type shown in FIGS. 5 through 7 (i.e., a mold-style device), in which recoating is performed by filling the recoating resin into a mold. Another example of a conventional device is shown in FIG. 8, in which a recoating resin that fills a die is applied to the optical fiber.
A recoating device 10 shown in FIGS. 5 and 6 has two upper and lower molds, 11A and 11B which are typically being made of light transmitting quartz glass. These molds 11A, 11B are provided between a base 12 and a lid 13 which is attached to the base 12 so as to permit opening and closing thereof. The recoating resin (typically a UV curable resin) is filled into respective semicircular mold grooves 16 of the molds 11A, 11B via resin filling ports 14, 15 which are formed in the lid 13 and upper mold 11B, and a resin filling route R which is formed in the upper mold 11B.
As shown in FIG. 6, the width of each mold groove 16 is designed to be longer than a bare fiber portion 1a of an optical fiber 1 which was joined using heat fusion for example.
Accordingly, when this optical fiber 1 is set in place in the respective mold grooves 16 of the molds 11A, 11B, a nearly closed cylindrical space is formed inside the mold grooves 16 around bare fiber portion 1a. The recoating resin is then filled into this space, forming a recoated sheath 2, i.e., a molded portion. Furthermore, UV light is radiated at this time by a resin curing means 17, such as a UV lamp, which is provided on the base 12 side for example, and then, curing of the resin can be promoted and the rapid recoating can be performed.
A recoating device 20 shown in FIG. 7 is an improved version of the above-described recoating device 10. This recoating device 20 has the same molds 11A, 11B, and a light blocking layer 18, consisting of a metal layer or the like, is provided on an abutting surface of each mold 11A, 11B at areas other than where mold groove 16 is located. As a result, areas other than the mold grooves 16 are not irradiated with UV light, so that resin curing can be prevented in areas where it is not necessary. Accordingly, it is possible to obtain an excellently formed recoated sheath.
A recoating device 30 shown in FIG. 8 has a die 31 used for the recoating resin and a resin curing means 32 such as a UV lamp or the like. An optical fiber 1 having a bare fiber portion 1a is held in a vertical orientation by upper and lower clamps 33 of the recoating device 30.
In this state, the die 31 is set in position at the bare fiber portion 1a, and the resin curing means 32 is set in position below the die 31. Furthermore, the recoating resin (UV curable resin) 3 which has been melted is supplied in the die 31 and the die 31 is raised.
As a result, a recoated sheath 2 of approximately the same diameter as the outer diameter of a sheath 1b of the fiber is gradually formed from below. The resin curing means 32 is also raised as the die 31 is raised, and the recoating resin 3 is gradually cured from below using UV light irradiation. In this way, the bare fiber portion 1a can be recoated.
However, as shown in FIG. 9, in the recoating device 10 shown in FIGS. 5 and 6, a problem that a sufficiently strong joining cannot be achieved between the recoated sheath 2 and sheath 1b (i.e., boundary area A), so that cracks, etc., may arise from boundary area A when the optical fiber is used after this recoating procedure arises.
For this reason, a method has been proposed for improving joining strength as shown in FIG. 10, wherein a small diameter part 1c is formed to the sheath 1b, and the recoating resin is filled over this area as well so as to form an extending part 2a to the recoated sheath 2.
However, in this method, there is an increase in the number of processing steps for the small diameter part 1c, which is troublesome and decreases productivity.
A method has also been proposed for obtaining a wide joining surface in the recoating device 10 shown in FIGS. 5 and 6, wherein the inner diameter of the cylindrical holes formed by the respective mold grooves 16 is designed to 20-30 μm larger than the outer diameter of the sheath 1b, and, as shown in FIG. 11, a thin film portion 2b which is 20-30 μm in thickness is formed on the end portions of recoated sheath 2.
However, in this method, it is difficult to accurately position the optical fiber 1 itself in the center of the cylindrical hole. Namely, as shown in FIG. 12, it is often the case that the thin film portion 2b is positioned eccentrically. When this occurs, the thin film portion is more prone to breaking, leading to cracking in the recoated sheath 2.
In the recoating device 20 shown in FIG. 7, it is possible to prevent the generation of thin film projections (burrs) 2c such as shown in FIG. 13 to portions of recoated sheath 2, since UV irradiation of unnecessary areas is prevented by the light blocking layer 18. However, in reality a small interval of space is unavoidably generated in the area between the inner surface of the cylindrical hole formed by the mold grooves 16 and the outer periphery of the sheath 1b. Moreover, since the light blocking layer 18 is not formed over the entire length to the ends of the respective mold grooves 16, it is difficult to prevent the formation of unnecessary thin film portion 2d as shown in FIG. 14. Furthermore, this thin film portion 2d becomes thinner approaching its ends and is not formed with a uniform shape. Thus, it easily peels and forms abnormal burrs, serving as a starting point for cracks in the recoated sheath 2.
In addition, in the case of the recoating device 30 shown in FIG. 8, it is difficult to form the outer diameter of the recoated sheath 2 to be uniform along its entire length. As a result, there are large variations in diameter, as shown in FIG. 15. For example, in a test sample using the optical fiber 1 in which the outer diameter of the sheath 1b was 0.25 mm, the maximum diameter portion D1 at the upper end of the recoated sheath 2 was 0.31 mm, while the minimum diameter portion D2 at the lower end of the recoated sheath 2 was 0.19 mm.